https://fredrikdeboer.com/2017/03/29/why-selection-bias-is-the-most-powerful-force-in-education/
Selection bias hides everywhere in education. Sometimes, in fact, it is deliberately hidden in education. A few years ago, Reuters undertook an exhaustive investigation of the ways that charter schools deliberately exclude the hardest-to-educate students, despite the fact that most are ostensibly required to accept all kinds of students, as public schools are bound to. For all the talk of charters as some sort of revolution in effective public schooling, what we find is that charter administrators work feverishly to tip the scales, finding all kinds of crafty ways to ensure that they don’t have to educate the hardest students to educate. And even when we look past all of the dirty tricks they use – like, say, requiring parents to attend meetings held at specific times when most working parents can’t – there are all sorts of ways in which students are assigned to charter schools non-randomly and in ways that advantage those schools. Excluding students with cognitive and developmental disabilities is a notorious example. (Despite what many people presume, a majority of students with special needs take state-mandated standardized tests and are included in data like graduation rates, in most locales.) Simply the fact that parents typically have to opt in to charter school lotteries for their students to attend functions as a screening mechanism.